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A Preliminary Disease Survey in the Wild Nile Crocodile (Crocodylus Niloticus) Population in the Okavango Delta, Botswana

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A Preliminary Disease Survey in the Wild Nile Crocodile (Crocodylus Niloticus) Population in the Okavango Delta, Botswana Article — Artikel A preliminary disease survey in the wild Nile crocodile (Crocodylus niloticus) population in the Okavango Delta, Botswana A J Lesliea*, C J Lovelya and J M Pittmanb MATERIALS AND METHODS ABSTRACT The objective of this study was to conduct a preliminary survey of diseases that might be Study site present in the wild Nile crocodile population in the Okavango Delta, Botswana. Blood Botswana’s Okavango Delta, the samples were collected from crocodiles ranging in size from 34.0 cm to 463.0 cm total length. world’s biggest Ramsar site (a wetland of Samples were examined for blood parasites and underwent a haematological analysis. international importance), is a large wet- Before release the crocodiles were examined for various clinical abnormalities. Of the 144 land within the Kalahari Desert, covering crocodiles examined, none were visibly sick or displayed any signs of disease. No antibodies an area of approximately 16 000 km2 in to were detected. was present in 55.3 % of blood Mycoplasma crocodyli Hepatozoon pettiti the dry season and increasing to over smears examined, but there was no significant difference in any of the haematological 22 000 km2 with the annual flood. The values between the infected and uninfected crocodiles, and a high prevalence of Hepatozoon 2 infection is not uncommon in other species. Only 7.6 % of the examined crocodiles were 111 250 km active catchment area falls infested with leeches. Further research is required for several of the crocodilian diseases, in entirely within Angola. Owing to the particular to elucidate the role of wild crocodilians as reservoirs of infection. geology of the catchment, the incoming water is low in nutrients and sediment32. Keywords: Nile crocodile, Crocodylus niloticus, Okavango Delta, disease survey. The Okavango River flows through Leslie A J, Lovely C J, Pittman J M A preliminary disease survey in the wild Nile croco- Namibia briefly before entering Botswana dile (Crocodylus niloticus) population in the Okavango Delta, Botswana. Journal of the and forming a broad floodplain, the South African Veterinary Association (2011) 82(3): 155–159 (En.). Department of Conservation Panhandle. An estimated 40 % of incom- Ecology & Entomology, University of Stellenbosch, Private Bag X1, Matieland, 7600 South ing water leaks into the surrounding Africa. swamps by the time the river leaves the Panhandle. The remaining 60 % is distrib- INTRODUCTION no other clinical signs and therefore uted down 3 main channels, which fan Since the development of the crocodile are not noticed to be unhealthy until out to form the Delta. The Okavango farming industry in the 1980s, a number they are found dead. Diseases causing Delta consists of permanent and seasonal of studies have been carried out world- visible external lesions, for example swamp, which is inundated during the wide on diseases in farmed croco- pox virus infection, are the obvious annual flood32. diles6,7,12,13,20,27,28. Very little, however, is exception. The northern part of the Delta is charac- known about the diseases of wild croco- (iv)Clinical examination has several limi- terised by shallow water, flooded grass- dilians, particularly wild Nile crocodiles. tations: cardiac and respiratory rates lands, ox-bow lakes and lagoons mostly Wild crocodiles are often difficult to study are variable according to conditions, interconnected by narrow waterways. owing to the remoteness of the areas in and are strongly influenced by capture Only a few main channels lined by tall which they occur. stress. Because crocodiles are poikilo- reeds (mainly Phragmites australis) carry The initial objective of this study was to thermic, body temperature cannot be the remainder of the Okavango’s water determine disease prevalence in the wild used as a diagnostic tool. southwards through the Delta. The per- Nile crocodile population in the Okavango (v) Sick/weak crocodiles are unlikely to manent and seasonal swamp together Delta. However, several factors made this survive for long due to predation. form a unique ecosystem and provide a difficult: Therefore, the likelihood of encoun- high-quality habitat for a great many (i) Very few serological tests have been tering these individuals in a capture species. As a keystone species, the Nile developed for crocodile diseases. and release survey is low. crocodile, helps maintain the fragile Consequently a serological survey is (vi)Owing to research permit limitations balance within this ecosystem. This of very limited value. no crocodiles could be sacrificed for includes selective predation on various (ii) Virus isolation is unavailable due to histopathological and parasitological fish species,10,42 recycling of nutrients and the lack of crocodile cell lines in veteri- examination. maintenance of wet refugia in times of nary diagnostic laboratories. In view of these limitations, it was pos- drought48. Crocodiles are unevenly dis- (iii)Sick crocodiles are difficult to identify tulated that a low incidence of diseases tributed throughout the Delta with the by observation. Under farmed condi- would be found in the Okavango popula- majority of the breeding population tions they often stop eating but show tion during a capture and release survey. occurring in the 120 km long Panhandle17. However, it was possible to do a more aDepartment of Conservation Ecology & Entomology, accurate survey for mycoplasmosis and Study animal University of Stellenbosch, Private Bag X1, Matieland, 7600 South Africa. haemogregarine infection thanks to the The Nile crocodile, Crocodylus niloticus, bJohannesburg Zoo, Private Bag X13, Parkview, 2122 availability of diagnostic tests. In addi- is the most widespread and abundant of South Africa. tion, ectoparasites, in the form of leeches, the 3 crocodile species that occur in Africa. *Author for correspondence. E-mail: [email protected] were easy to find, remove and count per It occurs throughout the continent south Received: July 2011. Accepted: August 2011. individual. of the Sahara in a variety of wetland habi- 0038-2809 Jl S.Afr.vet.Ass. (2011) 82(3): 155–159 155 tats, including coastal areas41,46. His- syringe was used, depending on the size into standard microhaematocrit tubes torically its distribution in southern Africa of the crocodile, and the blood was trans- and centrifuged for 5 min at 12 000 g. extended as far south as the Eastern Cape ferred directly into a lithium heparin tube. Total red cell counts (RCC) were per- Province during the past 100–200 years42. Blood smears were made from whole formed both manually and automatically Nile crocodiles are ectothermic and blood using the cover slip method24. Fol- using an electronic particle counter. The regulate their body temperature lowing blood collection, each crocodile automated counts were made using a behaviourally by moving between was measured (total length (TL) and Beckman Coulter Ac*T Series haematology sun-exposed sandbanks and the water. snout-to-vent length (SVL) using a flexible analyser (Coulter SA). The manual counts Typical adult lengths are around 3.5 m, measuring tape, ±1 mm). It was then were made using Natt and Herrick’s solu- but the males can grow up to 5 m18. Sexual weighed using a harness placed around tion. A 1:200 dilution was made by draw- maturity is reached from 2.9 m total the forelimbs and a Pesola spring balance. ing blood up to the 0.5 mark on a red length for males, and 2.2 m for females10. Each crocodile was sexed by cloacal exam- blood cell diluting pipette, then filling the Nesting occurs in a hole in the ground ination of the cliteropenis23,31 and the pipette to the 101 mark with Natt and and on average 50 eggs are laid. Nile croc- entire body was examined for clinical Herrick’s solution9. The diluted blood odiles exhibit temperature-dependent abnormalities including bite wounds, was then used to charge both counting sex determination31. Hatchlings emerge skin lesions, conjunctivitis, joint swelling chambers of an improved Neubauer after an incubation period of approximately and poor condition. Dorsal and lateral haemocytometer (Hawksley and Sons, 90 days in early to mid summer, and body surfaces were also examined for the Lancing, UK). After 5 mins in a humidity parental protection occurs39,40,41. As with presence of leeches. Leeches were removed chamber the red cells were counted in the other crocodilian species, a high mortality by means of a pair of tweezers, counted 4 corner cells and central cell of the central rate is experienced in their 1st year of life and stored in 70 % ethanol for later identi- large square of the counting chamber. due primarily to predation45. fication. This was repeated on the second chamber and the average multiplied by 10 000 to Capture methods Sample processing obtain the total red cell count per microlitre. One hundred and forty-four crocodiles On return to the field laboratory 1.0 m Haemoglobin concentrations (Hb) were were captured in the Panhandle of the of blood was transferred to an Eppendorf determined using a Beckman Coulter Okavango during summer (February tube for haematological analysis. The Ac*T Series haematology analyser (Coul- 2005). Capture was carried out using remaining blood was centrifuged using a ter SA). 2 methods. At night, using a 4.8 m manual desktop centrifuge and the Red blood cell indices were calculated flat-bottomed aluminium motor boat, plasma frozen for serology. If the volume using standard equations24 crocodiles were located with a spotlight of the blood sample was small it was allo- Mean cell volume: which revealed the eyes glowing red. The cated for either Hepatozoon examination MCV (fl) = PCV/RCC beam of light was then kept focused on or serology. the crocodile’s eyes, making it possible to Thirty-eight samples were examined for Mean cell haemoglobin: approach the animal by boat. Crocodiles blood parasites and underwent haemato- MCH (pg) = Hb (g/d ) × 10 / RCC estimated to be smaller than 1.2 m total logical analysis.
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